Adjust oven rack to the middle position and preheat to 400°F (200°C). Line a baking sheet with parchment paper.
Let salmon rest at room temperature for 15 minutes. Pat the fillets completely dry with a paper towel.
Place salmon on the baking sheet. Drizzle with olive oil and season with kosher salt and black pepper.
In a small bowl, combine the softened butter, minced garlic, chopped dill, parsley, lemon zest, and optional red pepper flakes. Mix until incorporated.
Spread the lemon herb butter mixture evenly over the top of each salmon fillet. Top each with 1-2 thin slices of lemon.
Bake for 12-18 minutes, until the salmon flakes easily with a fork or an instant-read thermometer reads 130-135°F in the thickest part.
Remove from the oven, squeeze the remaining lemon juice over the top, and let rest for 3-5 minutes before serving.
Notes
For best results, use an instant-read thermometer to check for doneness. Pull the salmon from the oven at 130°F for a perfect medium after resting.